This invention relates to a method of, and apparatus for, producing control signals to control a distortion reducing mechanism, such as may be used to linearise the output of a non-linear power amplifier.
A known control scheme for an amplifier predistortion arrangement comprises, as described in WO 99/45640, a mixer (e.g. 685 in that document) for multiplying a reference signal with a feedback signal derived from the output of the amplifier, which latter signal contains residual distortion. The mixing process is equivalent to a correlation process in that any component in the feedback signal which is equivalent in frequency to a component in the reference signal is mixed down to baseband and contributes to the DC component (i.e. at 0 Hz) of the mixer output. The mixer output is then integrated to remove AC components therein and is then used as a feedback control signal for the predistorter. That is, the DC component is isolated as the wanted signal. In the mixer, the type of detection performed is known as coherent detection since the wanted signal always appears at the same frequency, 0 Hz (unless, that is, the feedback signal supplied to the mixer has been frequency shifted by a local oscillator (LO), in which case the wanted signal always appears at the LO frequency.) This is distinct from incoherent conversion, where the wanted signal component will not always be mixed down to the same frequency.
In the control scheme described above, there are problems will DC offsets and offset drift in the correlation process. This is due to the correlation processes being performed in the analogue domain, resulting in a DC output. Ideally, this DC level would be directly proportional to the quantity being measured, (e.g. the amount of residual intermodulation distortion present), and would result in a zero volt output when the quantity has been minimised, (i.e. eliminated).
Unfortunately, analogue correlators (e.g. mixers or multipliers) more typically have a DC offset and hence their output will not fall to zero when the correlation result is minimised. In addition, this offset value will drift with time, temperature changes and input signal level changes, and hence it is usually difficult to use subtraction to cancel the offset with any degree of accuracy. The presence of such offsets and their fluctuating nature limits the achievable linearisation performance of the predistorter.
According to a first aspect, the invention provides a method of reducing the appearance of distortion in an output signal which a signal handling means produces in response to an input signal, the method comprising: sampling both the input and output signals; frequency shifting one of the sampled signals by a frequency offset amount; converting, by detection, the frequency-offset sampled signal and the other sampled signal to baseband signals, processing the baseband signals to produce control signals; and predistorting the input signal under control of the control signals.
According to a second aspect, the invention also provides apparatus for reducing the appearance of distortion in an output signal which a signal handling means produces in response to an input signal, the apparatus comprising means for sampling both the input and output signals, means for frequency shifting one of the sampled signals by a frequency offset amount, means for converting, by detection, the frequency offset sampled signal and the other sampled signal to baseband signals, means for processing the baseband signals to produce control signals, and means for predistorting the input signal under control of the control signals.
The invention advantageously uses detector-based down-conversion, rather than local oscillator based down conversion with its associated disadvantages. In the present context, frequency conversion to baseband by detection relates to frequency conversion to baseband without the use of local oscillator signals. It can either involve implicit multiplication (by, e.g. square-law detectors which output the square of their input), or can be achieved by correlation of, for example, an input signal with a related output signal (using, e.g. a mixer, a multiplier, or a non-linear device such as a diode). No additional or auxiliary signals (such as local oscillators and the like) are required to perform the frequency conversion.
Another advantage of the invention is that it produces baseband signals suitable for conversion to digital signals to allow the control signals to be produced in the digital domain in, for example, a digital signal processor. This means that the process of producing the control signals may be performed using digital correlation thus avoiding DC-offsets and DC-offset drift associated with mixers and multipliers used in analogue correlation processors.
In a preferred embodiment, the sampled input signal is multiplied with itself to produce a first reference signal which is one order higher than a target distortion component of a particular order in the output signal. The sampled input signal may be multiplied with the sampled output signal to produce a second reference signal. The first and second reference signals may be multiplied together to produce a third reference signal at the offset frequency. The third reference signal may be multiplied with a signal at the offset frequency in the digital domain to produce DC signals for controlling the predistortion process.
In an alternative embodiment, however, the third reference signal may be multiplied with a signal at the offset frequency on the analogue domain.
As mentioned above, producing the DC control signals in the digital domain may substantially eliminate the problems of DC offset and DC offset drift associated with analogue methods of producing control signals. The target distortion component may be a third order distortion component, and the control signals developed by correlating with the target distortion component may be used to control the predistorter in the suppression of the target distortion component or a wider spectrum of distortion.
The preferred embodiment may also include multiplying the sampled input signal with itself to produce further reference signals, each of which is one order higher than a corresponding target distortion component of a specific order appearing in the output signal, and multiplying each further reference signal with the second reference signal to produce modified further reference signals at the offset frequency. The modified further reference signals may be multiplied with a signal at the offset frequency to produce DC control signals for controlling the predistortion process to substantially eliminate distortion appearing in the output signal and corresponding to the respective target distortion components. In this way, the system can be extended to individual control of the predistorter to combat distortion appearing at individual target distortion components.
In a preferred embodiment, the predistortion process is diode, FET, Bipolar transistor, dual-gate FET or mixer based.